1. Field of the Invention
The present invention relates to a power output apparatus, and more specifically to an power output apparatus for outputting power to a drive shaft.
2. Description of the Prior Art
In known power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically linked with a drive shaft, which connects with a rotor of a motor, by means of an electromagnetic coupling, so that the power output from the engine is transmitted to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). In this power output apparatus, the vehicle starts driving by the function of the motor. When the revolving speed of the motor reaches a predetermined level, the power output apparatus supplies an exciting electric current to the electromagnetic coupling to crank the engine and carries out a fuel supply into the engine and ignition with a spark, thereby starting the engine. After the engine starts, the power output from the engine is transmitted to the drive shaft via the electromagnetic coupling, in order to continue driving the vehicle. When the power transmitted to the drive shaft via the electromagnetic coupling is insufficient for the required power on the drive shaft, the motor is driven to supplement the insufficiency. The electromagnetic coupling regenerates electric power according to a slip of its electromagnetic connection while the power is transmitted to the drive shaft. The regenerative electric power is stored in a battery as electric power used for starting the vehicle or is used as a power of the motor to supplement the insufficiency of the power on the drive shaft.
In this known apparatus, the electromagnetic coupling can transmit part of the power output from the engine to the drive shaft and regenerate the residual power as electric power, irrespective of the driving point of the engine. The driving point of the engine can thus be arbitrarily set to have a high efficiency.
Even when the engine can be driven at the driving point of high efficiency, however, this conventional apparatus still has some problems; that is, a low efficiency of the whole power output apparatus and the large-sized electromagnetic coupling and motor. The efficiency of the whole power output apparatus depends upon not only the efficiency of the engine but the efficiencies of the electromagnetic coupling and the motor and the charge-discharge efficiencies of the battery. The low efficiencies of these constituents lead to a low efficiency of the whole power output apparatus. The power required for the drive shaft is generally different from the power output from the engine that is driven at the driving point of high efficiency, so that torque conversion of the power by means of the electromagnetic coupling and the motor is required. As discussed above, torque conversion involves the power directly transmitted to the drive shaft via the electromagnetic coupling and the power output to the drive shaft by supplying the electric power regenerated by the electromagnetic coupling to the motor. The latter torque conversion is implemented via the form of electrical energy. The large electrical energy causes a large energy loss and lowers the efficiency of the whole power output apparatus. The greater proportion of the latter torque conversion via the form of electrical energy requires the large capacity of electromagnetic coupling and the motor and thereby makes these constituents rather bulky.